The present invention relates to an illuminating device suited for back light units used for high luminance liquid crystal display device.
As a back light source used for liquid crystal monitor for a personal computer or a liquid crystal TV set that requires high luminance and high efficiency, a cold cathode low pressure discharge lamp is generally used. The cold cathode low-pressure discharge lamp encloses mercury and rare gas as a discharge medium inside a tubular glass lamp vessel. A phosphor layer is formed on the inner wall of the glass lamp vessel, and the cold cathodes are provided in the glass vessel at its both ends. A high frequency sine wave voltage is supplied between the cold cathodes provided by a high frequency power source to light the low-pressure discharge lamp.
Now, a high luminance of 450 cd/m or higher is required for the back light source for liquid crystal TV set. In order to fulfill the requirement, a right under type back light unit is used, in which more than ten lamps are provided right under the light guide plate. When the cold cathode low-pressure discharge lamp is used for this type of back light unit, one inverter, which is a high frequency voltage source, can light only one lamp. Therefore, the inverters of as many as the number of the lamps are necessary, which results in high cost for inverters.
In order to solve the problem, a low cost back light source for liquid crystal TV set has been developed, in which a dielectric barrier discharge type low pressure discharge lamp (called as a dielectric barrier discharge lamp) is provided having outer electrodes made of an electrically conductive layer formed on an outer surface of end portions of the tubular glass lamp vessel.
The dielectric barrier discharge lamp has a phosphor layer on the inner wall of a tubular glass lamp vessel enclosing mercury and rare gas inside and outer electrodes formed on an outer surface of end portions of the glass lamp vessel. With the lamp described, a plurality of lamps can be lighted with one inverter at a time, reducing the cost of inverter in the back light unit.
However, such dielectric barrier discharge lamp enclosed with mercury had the following problems. When the discharge lamp is lighted by an inverter which provides a sine wave voltage having a frequency of 50 KHz at its output terminals with one end of the output terminal being grounded, a corona is discharged from the outer electrode of the lamp connected with the high voltage output terminal of the inverter due to the high lamp voltage exceeding 2000 Vrms and the corona result in generation of ozone.
Further, when such dielectric barrier discharge lamp enclosed with mercury is driven by a square wave voltage, which is used for lighting a dielectric barrier discharge lamp enclosed with xenon and free of mercury, there was a problem that the efficiency became worse compared with the lighting by sine wave voltage.
On the other hand, recently, a so-called narrowed frame picture, which means a display panel having a narrow frame area at a periphery of the picture, is required for liquid crystal display units, in order to make the viewing area in display panel as large as possible. It is required for a structure of the discharge lamp for the back light unit satisfying such demand to make the electrode length shorter on the both ends of the glass tube to make the effective length of the discharge lamp longer.
In general, making the length of the outer electrode of the dielectric discharge lamp short decreases the tube current and thus decreases the luminance of the lamp. Therefore, it is desired to obtain a sufficient tube current even when the outer electrode length is short. There are two measures to overcome this problem. One is to raise the tube voltage; another is to increase the oscillating frequency. The latter of the two is difficult to realize because of the limit in the transformer. Therefore, the former measure of raising the tube voltage has been adopted in the past.
However, for example, to obtain a tube current of 4 mA in the dielectric barrier discharge lamp having a reduced length electrode of 10 mm long, some 1.7 times voltage as high as the voltage must be applied, which voltage is necessary for obtaining 4 mA tube current in a dielectric barrier discharge lamp having an electrode length of 2.0 mm. As the result, the tube voltage exceeds 2000 Vrms, and the problem that ozone is generated from the electrode due to the corona discharge as described above.
Further, it is necessary to raise the tube voltage in order to increase luminance of the discharge lamp, since the oscillating frequency is difficult to increase owing to the characteristic limit of the transformer. Thus the ozone is generated more actively.
As mentioned above, there has been no other means than raising the tube current with increased length of the electrode for the dielectric barrier discharge lamp to increase the luminance of the back light illuminating device. However, this means makes the effective light emitting length shorter and cannot fully meet the demand for the narrow frame picture display.
The present invention has been made to overcome such conventional technical problems and thus it is an object of the present invention to supply an illuminating device, with which a dielectric barrier discharge lamp enclosed with mercury is driven to light under the best conditions.